This invention relates to liquid level detectors of the type having a column of sensors capable of being operated by a signal or field from an adjacent floating signal or field generator, and more particularly pertains to such a level detector having a resolution that is smaller than the spacing of the sensors.
One prior art liquid level gauge is comprised of a string of series connected magnetic field actuated reed relays. A resistor is connected across each relay. The relay string and resistors are mounted in a protective tube which is mounted vertically in the tank. A magnet is mounted to an annular float that is adapted to be slidably movable about the tube in a vertical direction. The total resistance of the string is taken as a measure of the liquid level. This total resistance is inversely proportional to the number of reed relays whose contacts are closed and provides an analog measure of the liquid level. Such level detectors are especially insensitive to small level changes when the tank is from about half full to full of liquid.
A level detector of improved accuracy is described by Nagy in his U.S. Pat. No. 4,361,835 issued Nov. 30, 1982, and assigned to the same assignee as is the present invention. That detector employs a vertical column of Hall-cells and associated Schmitt trigger circuits that are scanned by turning on one Hall-Schmitt switch at a time in sequence. Their outputs are connected together to an output line. A floating magnet thus causes an adjacent Hall-cell and associated switch to produce a signal on the output line a number of scan time intervals later than the start of a scan cycle. This number of scan time intervals corresponds to the position in the column of that magnet-actuated Hall-sensor and can be very easily converted into a binary or other code that is suitable for data storage or display. This measure of liquid level has a characteristic resolution that is equal to the spacing of the Hall-cells in the column, at every liquid level. To obtain a smaller resolution requires the use of a greater number of Hall-cells and associated circuitry that entails a proportional increase in cost.
It is an object of the present invention to provide a multiple Hall-cell type liquid level detector that has a substantially smaller resolution than the spacing of the Hall-cells and thus relatively lower cost for a detector of a given resolution.